Impact of Ontogenetic Changes in Branchial Morphology on Gill Function inArapaima gigas

Abstract

Soon after hatching, the osteoglossid fish Arapaima gigas undergoes a rapid transition from a water breather to an obligate air breather. This is followed by a gradual disappearance of gill lamellae, which leaves smooth filaments with a reduced branchial diffusion capacity due to loss of surface area, and a fourfold increase in diffusion distance. This study evaluated the effects these changes have on gill function by examining two size classes of fish that differ in gill morphology. In comparison to smaller fish (approximately 67.5 g), which still have lamellae, larger fish (approximately 724.2 g) without lamellae took up a slightly greater percentage of O2 across the gills (30.1% vs. 23.9%), which indicates that the morphological changes do not place limitations on O2 uptake in larger fish. Both size groups excreted similar percentages of CO2 across the gills (85%-90%). However, larger fish had higher blood PCO2 (26.51.9 vs. 16.51.5 mmHg) and HCO3(-) (40.2 +/- 2.9 vs. 33.6 +/- 4.5 mmol L(-1)) concentrations and lower blood pH (7.58 +/- 0.01 vs. 7.70 +/- 0.04) than did smaller fish, despite having lower mass-specific metabolisms, suggesting a possible diffusion limitation for CO2 excretion in larger fish. With regard to ion regulation, rates of diffusive Na+ loss were about 3.5 times higher in larger fish than they were in smaller fish, despite the lowered branchial diffusion capacity, and rates of Na+ uptake were higher by about the same amount despite 40% lower activity of branchial Na+/K+-ATPase. Kinetic analysis of Na uptake revealed an extremely low-affinity (K(m) = 587.9 +/- 169.5 micromol L(-1)), low-capacity (J(max) = 265.7 +/- 56.8 nmol g(-1) h(-1)) transport system. These data may reflect a general reduction in the role of the gills in ion balance. Renal Na+/K+-ATPase activity was 5-10 times higher than Na+/K+-ATPase activity in the gills, and urine: plasma ratios for Na+ and Cl(-) were very low (0.001-0.005) relative to that of other fish, which suggested an increased role for dietary salt intake and renal salt retention and which was representative of a more "terrestrial" mode of ion regulation. Such de-emphasis of branchial ion regulation confers greatly reduced sensitivity of diffusive ion loss to low water pH. Ammonia excretion also appeared to be impacted by gill changes. Rates of ammonia excretion in larger fish were one third less than that in smaller fish, despite larger fish having blood ammonia concentrations that were twice as high.

Implications for Osmorespiratory Compromise by Anatomical Remodeling in the Gills of Arapaima gigas

“…Almost half of the MRCs are immature cells with low immunoreactivity against Na 1 /K 1 -ATPase (light MRCs) and are not in contact with the environment. This result was clearly evidenced by the lower MRC frequency at the epithelial surface compared to the total MRC frequency in the gill epithelium and agrees with the differences in the gill Na 1 /K 1 -ATPase activity in small and large fish reported by Gonzalez et al (2010). In A. gigas, the kidney most likely plays an important role in ion and pH regulation Gonzalez et al, 2010).…”

“…Thus, the morphological changes in the gills of A. gigas may reduce the capacity of O 2 to diffuse through the lamella. Previous data showed that the percentage of O 2 uptake by the gills of fish weighing between 2,000 and 3,000 was only 23% (Stevens and Holeton, 1978); however, some physiological adjustments for compensating the gill morphological restrictions may occurs in fish smaller than 1,000 g, as Gonzalez et al (2010) reported higher O 2 uptake percentage in 724.2 g fish (30%) than in 67 g fish (24%).…”

“…Increased thickness in the lamellar epithelium in Oncorhynchus mykiss treated with cortisol or growth hormone (Bindon et al, 1994) and in C. auratus in which ILCM was induced by acclimation at low temperature (Tzaneva et al, 2011) increases the arterial blood partial pressure of CO 2 (P CO2 ). Partitioning respiratory physiological data from 1,000 to 2,000 g A. gigas (Randall et al, 1978;Stevens and Holeton, 1978;Brauner and Val, 1996) showed that 70% of CO 2 was excreted by the gills and, more recently, Gonzalez et al (2010) reported 85% of CO 2 excretion across the gills in different size of fish; however, the higher blood P CO2 and HCO 3 2 concentrations as well as the lower blood pH in larger fish provide evidence that there are some constraints for CO 2 excretion in large (724 g) fish.…”

“…Conversely, CO 2 is primarily excreted by the gills (63-78%) and, to a lesser extent, by the swim bladder (15%) and kidney (6%) Stevens and Holeton, 1978;Brauner and Val, 1996). At low magnification, scanning electron microscopy (SEM) revealed welldeveloped lamellae in the gill filaments of 10-100 g fish (22-25 cm; 2-3 months old) but not in those of 724 g and 1,000 g fish (Brauner et al, 2004;Gonzalez et al, 2010). The lamellar organization of a 100 g A. gigas is similar to that of other teleosts, and its respiratory surface area is similar to those of facultative air-breathing fish of a similar size (Costa et al, 2007); the average respiratory surface area of the swim bladder exceeded that of the gills by a factor of 2.8 indicating the importance of the swim bladder for respiration, even in juvenile fish (Fernandes et al, 2012).…”

“…The lamellar organization of a 100 g A. gigas is similar to that of other teleosts, and its respiratory surface area is similar to those of facultative air-breathing fish of a similar size (Costa et al, 2007); the average respiratory surface area of the swim bladder exceeded that of the gills by a factor of 2.8 indicating the importance of the swim bladder for respiration, even in juvenile fish (Fernandes et al, 2012). Data concerning ion regulation has demonstrated that the rate of diffusive Na 1 loss is higher in 724 g fish (45 cm; 5 months old) than in 67 g fish (22 cm, 2 months old) (Gonzalez et al, 2010) and Brauner et al (2004) identified high density of mitochondria-rich cells (MRCs) along the outer cell layer of the filament epithelium of 1,000 g fish. However, the changes in the lamellar structure and the morphological processes involved in the gill remodeling of A. gigas during development are not yet known.…”